Microelectronic devices are typically fabricated, in part, by forming features on selected layers of a semiconductor wafer. The individual features are often formed by patterning a mask to protect selected portions of an underlying layer of material during subsequent processing steps. The mask of the present invention may be used in the fabrication of virtually any type of microelectronic device, but it is particularly useful in the fabrication of field emission displays ("FEDs") in use or proposed for use in computers, television sets, camcorder viewfinders, and a variety of other applications.
FEDs are one type of flat panel display in which a baseplate with a generally planar emitter substrate is juxtaposed to a faceplate with a substantially transparent display screen. The baseplate has a number of emitters formed on the emitter substrate that project from the emitter substrate towards the faceplate. The emitters are typically configured into discrete emitter sets in which the bases of the emitters in each emitter set are commonly connected. The baseplate also has an insulator layer formed on the emitter substrate and an extraction grid formed on the insulator layer. A number of holes are formed through the insulator layer and extraction grid in alignment with the emitters to open the emitters to the faceplate. In operation, a voltage differential is established between the extraction grid and the emitters to extract electrons from the emitters.
The display screen of the faceplate is coated with a substantially transparent conductive material to form an anode, and the anode is coated with a cathodoluminescent layer. The anode draws the electrons extracted from the emitters through the extraction grid and across the vacuum gap between the extraction grid and the cathodoluminescent layer of material. As the electrons strike the cathodoluminescent layer, light emits from the impact site and travels through the anode and the glass panel of the display screen. The emitted light from each of the areas becomes all or part of a picture element.
One objective of field emission displays is to produce a desired brightness of light in response to the emitted electrons. The brightness at each picture element depends, in part, upon the density of emitters in the emitter sets corresponding to each picture element. In general, it is desirable to have a constant emitter density from one emitter set to another, and also from one area in an individual emitter set to another. Thus, it is desirable to space the emitters apart from one another by a substantially uniform distance, and to make the emitters substantially the same size and shape.
One emerging method for forming emitters is to randomly distribute a number of beads on a hard oxide layer that has been deposited over the emitter substrate. The beads may be distributed across the surface of the oxide layer by depositing a solution in which the beads are suspended onto the oxide layer, spinning the substrate to spread the solution, and evaporating the liquid to leave the beads on the oxide layer. The beads may also be distributed across the surface of the oxide layer by a dry dispensing method in which a dry mixture of beads is propelled towards the oxide layer in a jet of air or inert gas. The mixture is then allowed to settle on the oxide layer to form a mask of randomly distributed particles on the surface of the oxide layer. The unmasked areas of the oxide layer are then etched with an etchant selective to the material of the oxide layer to form a random distribution of island-like oxide areas under the beads. After the beads are removed from the oxide layer, the emitters are formed under the island-like areas of oxide by isotropically etching the substrate.
One problem with this method for patterning the oxide layer is that the island-like oxide areas, and thus the emitters, may not have a uniform size and shape. The desired diameter of the base of each emitter is generally the diameter of a single, isolated bead. However, in this patterning method, the beads often agglomerate into clusters that remain intact as they are distributed across the surface of the substrate. It will be appreciated that clusters of beads produce larger, irregular-shaped islands of oxide which result in larger, irregular-shaped emitters. As a result, the emitters produce by this emitter pattering method may not have a uniform size and shape.
Another problem with this emitter patterning method is that the emitters may not be uniformly spaced apart from one another. Since the beads are distributed randomly across the surface of the oxide layer, it is difficult to control the space between the beads. Thus, the space between the emitters produced by this emitter patterning method may vary significantly from one area on the display to another.
In light of the problems associated with patterning an oxide layer to form emitters, it would be desirable to develop a mask that produces emitters with a substantially uniform size and shape, and controls the space between the emitters.